1. Field of the Invention
This invention relates to a high resolution uniform speed scanning lens for use in an information processing terminal apparatus for recording or reading images.
2. Description of the Prior Art
In FIG. 1 of the accompanying drawings, a perspective view of a laser recording apparatus, which is an embodiment of an information processing terminal apparatus, is shown as an example of apparatus using a uniform speed scanning lens. In FIG. 1, a laser beam oscillated by a laser oscillator 1 enters a light modulator 2. The beam, subjected to modulation by the light modulator in accordance with an information signal to be recorded has its beam diameter expanded by a beam expander 3, but remains a parallel beam, and is incident on a light deflector 4. The beam is deflected at an constant angular speed by the light deflector, which may be, for example, a rotatable polygonal mirror or a galvano mirror. The deflected beam is imaged on a recording medium 6 (for example, a sensitive film) by an imaging lens 5.
In the imaging lens, like an ordinary photographic lens, the image height y is proportional to the tangent of the incident angle .theta.. That is, if a y=f tan .theta. lens is used (f is the focal length of the imaging lens), dy/d.theta.=sec.sup.2 .theta. and for a constant angular speed deflection, the beam spot on the imaging plane must move at a uniform speed. Therefore, if a lens in which the incident angle is proportional to the image height, namely, a lens having a characteristic of y=f.multidot..theta. (that is an f-.theta. lens) is used as the imaging lens, dy/d.theta.=f and, if the beam is deflected at a constant angular speed by the rotatable polygonal mirror, the spot will move at a uniform speed on the recording medium. That is, in order that the scanning beam may be imaged flatly and at a uniform speed on a scanning surface by the use of a deflector effecting an equal angular speed deflection, the imaging lens used must be a lens having a characteristic of Y=f.multidot..theta..
An example of a f-.theta. lens used in the conventional information processing terminal apparatus is disclosed in U.S. Pat. No. 4,056,307 and the cross-section of this lens is shown in FIG. 2 of the accompanying drawing.
In FIG. 2, reference numeral 1 designates a pupil which is comprised of a deflector such as a rotatable polygonal mirror. Reference numerals 2 to 7 denote cylindrical lenses each having a power in a plane parallel to the scanning direction (the direction horizontal relative to the plane of the drawing sheet), reference numeral 8 designates a cylindrical lens having a power in a plane perpendicular to the scanning direction (the direction perpendicular and vertical relative to the plane of the drawing sheet), and reference numeral 9 denotes an imaging plane (a scanned surface). Another example of an f-.theta. lens used in another information processing terminal apparatus, is described in `Three Optimized Designs For Flat Field Scanning Lenses` announced by R. E. Hopkins in p.110-p.114 of SPIE vol. 84 Laser Scanning Components & Techniques (1976), and the cross-section of this lens is shown in FIG. 3 of the accompanying drawing. In FIG. 3, reference numeral 10 designates a pupil, reference numerals 11 to 14 denote spherical lenses, and reference numeral 15 designates an imaging plane. A characteristic of these conventional f-.theta. lenses is that the F-number is as dark as F no=8. Where the f-.theta. lens is used in such a scanning system, a sufficiently large space must be provided between the deflector and the scanning lens because of the fact that the deflector is rotated. This is particularly so where the focal length of the scanning lens is short. However, in the conventional f-.theta. lens, for example, the lens disclosed in the aforementioned U.S. patent, t/f=0.4239, where t is the distance from the pupil (in this case, the deflecting surface of the deflector) to the first surface of the scanning lens and f is the focal length of the lens, and thus the spacing between the deflector and the scanning lens could not be said to be sufficient.